WO2024111638A1

WO2024111638A1 - Grain-oriented electromagnetic steel sheet and production method therefor

Info

Publication number: WO2024111638A1
Application number: PCT/JP2023/042040
Authority: WO
Inventors: 克高橋; 雅人安田; 宣郷森重; 直樹和田; 将嵩岩城; 尚茂木
Original assignee: 日本製鉄株式会社
Priority date: 2022-11-22
Filing date: 2023-11-22
Publication date: 2024-05-30

Abstract

This grain-oriented electromagnetic steel sheet has a silicon steel sheet, an oxide layer formed on the surface of the silicon steel sheet and comprising oxides of one or more of Mg, Al, and Si, and an insulating coating layer formed on the surface of the oxide layer. Oxides of one or more of Mg, Al, and Si having a circle-equivalent diameter of 0.1-3.0 μm are present at a density of 0.010-0.200 particles/μm2 in a range of 5 μm in the sheet thickness direction of the silicon steel sheet from the interface of the silicon steel sheet and the oxide layer, flat crystal grains are present on the surface side of the silicon steel sheet, the length of the grain boundaries of the flat crystal grains constituting the length of the interface of the silicon steel sheet and the oxide layer in a cross section in the sheet thickness direction is 50% or more, and a plurality of instances of linear thermal strain extending in a direction forming an angle of 80º-100º with respect to the rolling direction in the surface of the silicon steel sheet are formed at intervals of 1.0-20.0 mm with respect to the rolling direction.

Description

Grain-oriented electrical steel sheet and its manufacturing method

The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof.
This application claims priority based on Japanese Patent Application No. 2022-186166, filed on November 22, 2022, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheet is a soft magnetic material, and is mainly used as the iron core material of transformers. Grain-oriented electrical steel sheet is a steel sheet containing, for example, 2.00-6.00% Si, with the crystal orientation of the product highly concentrated in the {110}<001> orientation. Its magnetic properties require high magnetic flux density, represented by the B8 value, and low iron loss, represented by W17/50. In particular, there has been a growing demand in recent years for reduced power loss in transformers from the perspective of energy conservation, and there is an increasing demand for reduced iron loss in grain-oriented electrical steel sheet.

　Iron loss can be broadly divided into two loss components: hysteresis loss and eddy current loss. Eddy current loss can be further divided into classical eddy current loss and anomalous eddy current loss.

For example, known methods for reducing classical eddy current loss include increasing the electrical resistance of grain-oriented electrical steel sheets and reducing the thickness of silicon steel sheets that serve as base steel sheets.
However, these methods are undesirable because they reduce productivity, and furthermore, sufficient iron loss improvement effects cannot be obtained by these measures alone.

In order to reduce hysteresis loss, it is important to increase the magnetic flux density, and it is effective to control the crystal orientation in the steel sheet to an orientation close to the Goss orientation by cold rolling or controlling inhibitors.
For example, Patent Document 1 describes that by forming an oxide layer rich in silica by decarburization annealing, the decomposition and disappearance of the inhibitor is suppressed, and recrystallization of crystal grains having a crystal orientation close to the Goss orientation (hereinafter referred to as Goss orientation grains) can be stably caused.
According to the above method, although the magnetic flux density is improved, the number of recrystallized Goss-oriented grains is reduced, so that the number of Goss-oriented grains per unit area of the steel sheet is reduced. That is, the crystal grain size per Goss-oriented grain becomes larger, and as a result, the magnetic domain width of the 180° magnetic domain (hereinafter referred to as magnetic domain width) that contributes to the magnetic properties of the grain-oriented electrical steel sheet formed in the Goss-oriented grain becomes larger. In this case, even if the hysteresis loss is reduced by improving the magnetic flux density, the abnormal eddy current loss increases due to the increase in the magnetic domain width, so that the iron loss is offset, and there is a concern that the iron loss reduction effect commensurate with the improvement in magnetic flux density cannot be obtained.

For this reason, methods have been proposed to reduce abnormal eddy current loss by narrowing the magnetic domain width while still enjoying the effect of reducing hysteresis loss by improving magnetic flux density. A commonly used method is to periodically apply thermal strain to the surface of grain-oriented electrical steel in the rolling direction, and high energy sources such as lasers and electron beams are commonly used for this purpose.
As one example, Patent Document 2 discloses a method for manufacturing a grain-oriented electrical steel sheet in which magnetic domains are controlled by irradiation with laser light, the method comprising the steps of irradiating a surface of the grain-oriented electrical steel sheet with focused continuous wave laser light while scanning it in a direction inclined from the rolling direction of the grain-oriented electrical steel sheet, and repeating the steps while shifting the area scanned with the continuous wave laser light at a predetermined interval, wherein, when the average power of the continuous wave laser light is expressed as P (W), the scanning speed is expressed as Vc (mm/ ^s ), the predetermined interval is expressed as PL (mm), and the average irradiation energy density Ua is defined as Ua = P/(Vc × PL) (mJ/ ^mm2 ), the following are satisfied: ^1.0 mm ≦ PL ≦ 3.0 mm, and 0.8 mJ/mm2 ≦ Ua ≦ 2.0 mJ/mm2.
Patent Document 2 shows that it is possible to easily reduce iron loss in both the rolling direction and the width direction of a grain-oriented electrical steel sheet while ensuring high productivity.

Furthermore, Patent Document 3 discloses a method for manufacturing a grain-oriented electrical steel sheet in which linear closure domains are formed at approximately regular intervals and substantially perpendicular to the rolling direction of the steel sheet by scanning and irradiating the steel sheet with a continuous wave laser beam, thereby improving the iron loss characteristics.
Patent Document 3 discloses that a grain-oriented electrical steel sheet with reduced iron loss can be obtained by using a laser in a TEM00 mode in which the laser light intensity distribution in a cross section perpendicular to the beam propagation direction has the maximum intensity near the center of the optical axis, and by setting the focusing diameter d [mm] of the irradiation beam in the rolling direction, the scanning linear velocity V [mm/s] of the laser beam, and the average laser output P [W] within the ranges of 0<d≦0.2 and 0.001≦P/V≦0.012.

In order to reduce iron loss, improvements in magnetic domain control technology are required in addition to the magnetic flux density technology mentioned above. However, in recent years, progress in magnetic domain control technology has not kept pace with progress in high magnetic flux density technology, and there has been an issue in which iron loss reduction commensurate with the increase in magnetic flux density has not been fully achieved.

Japanese Patent Publication No. 62-151522 Japanese Patent No. 4669565 Japanese Patent No. 4510757

As mentioned above, there have been studies on improving magnetic flux density and achieving an iron loss reduction effect commensurate with the improvement, but these have not been sufficient to meet the increasingly high demands of recent years. Even so, so-called thermally strained magnetic domain control materials (hereinafter simply referred to as magnetic domain control materials), which are mainly suitable for stacked cores and are made by irradiating grain-oriented electrical steel sheets with lasers, electron beams, plasma, etc. to intentionally impart thermal strain to reduce the magnetic domain width and reduce iron loss, are not sufficient, and research is being conducted on room for reducing iron loss.
Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet having excellent magnetic properties in a magnetic domain control material, i.e., high magnetic flux density and low core loss commensurate with the magnetic flux density, and a manufacturing method thereof.

The inventors have mainly investigated improvements in the magnetic properties of magnetic domain control materials in grain-oriented electrical steel sheets suitable for use in stacked cores, i.e., improvements in magnetic flux density and reductions in iron loss. As a result, they have found that by forming crystal grains (hereinafter "flat crystal grains") that are flat and have a crystal orientation that deviates from the Goss orientation ({110}<001> orientation) by 10° or more on the surface side of the silicon steel sheet (base steel sheet) that grain-oriented electrical steel sheets are provided with, it is possible to energetically control the 180° magnetic domain width to a small state, and therefore eddy current loss can be reduced more than in the past even when thermal distortion is applied in the same way as in the past, and as a result, iron loss can be reduced even more.

The present inventors also investigated the influence of production conditions, and as a result, obtained the following findings.
That is, the Goss orientation, which develops high magnetic properties in grain-oriented electrical steel sheets, is highly accumulated by the abnormal grain growth phenomenon called "secondary recrystallization" that utilizes the pinning effect of the precipitates, which are called inhibitors, and are precipitated at the grain boundaries in the final annealing process of the manufacturing process. After the accumulation of the Goss orientation in the steel sheet is completed, that is, after the steel sheet surface is almost completely covered with Goss orientation grains, the inhibitor that has completed its role is decomposed and oxidized by the temperature rise in the latter half of the final annealing process and removed from the steel sheet. That is, it is not preferable that the decomposition and oxidation of the inhibitor occur before the Goss orientation is sufficiently accumulated in the steel sheet. Furthermore, by suppressing the decomposition and oxidation of the inhibitor to a higher temperature, it is possible to accumulate the Goss orientation to a higher degree, that is, to accumulate crystals closer to the ideal Goss orientation. Therefore, a method is used to increase the heat resistance of the precipitates that act as inhibitors.
The present inventors have found that, as a method for improving the heat resistance of the inhibitor, it is effective to make oxides capable of suppressing the decomposition and oxidation of the inhibitor during the subsequent finish annealing in the decarburization annealing process that is normally performed in the manufacture of grain-oriented electrical steel sheets, present on the steel sheet surface. Furthermore, they have found that by making oxides capable of suppressing the decomposition and oxidation of the inhibitor present on the steel sheet surface by utilizing the decarburization annealing process before the finish annealing, it is possible to generate flat crystal grains near the interface between the steel sheet surface oxide and the steel sheet.
The inventors also discovered that in order to generate more preferable flat crystal grains for improving magnetic properties, it is effective to form oxide particles more densely, thickly and uniformly on the surface side of the cold-rolled sheet that becomes the base steel sheet in the decarburization annealing process, and that in order to form oxide particles more densely, thickly and uniformly, it is effective to grind the cold-rolled sheet under specified conditions before the decarburization annealing process in order to remove reaction products with the surface of the steel sheet that inhibit uniform oxidation of the steel sheet surface during decarburization annealing.

In addition, grain-oriented electrical steel sheets may be irradiated with lasers, electron beams, plasma, etc. to intentionally impart thermal distortion and control magnetic domains. However, because the above-mentioned grain-oriented electrical steel sheets have a small magnetic domain width before magnetic domain control is applied, it has been discovered that by combining this technology with other technologies, a synergistic effect can be achieved, resulting in even more excellent magnetic properties, i.e., high magnetic flux density and reduced iron loss.

The present invention has been made in view of the above findings.
[1] A grain-oriented electrical steel sheet according to one embodiment of the present invention comprises a silicon steel sheet, an oxide layer formed on the surface of the silicon steel sheet and made of one or more oxides of Mg, Al, and Si, and an insulating coating layer formed on the surface of the oxide layer, wherein oxides of one or more of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm are present at 0.010 to 0.200 particles/μm within a range of 5 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer of the silicon steel sheet. and a density of ² , flat crystal grains are present on the surface side of the silicon steel sheet, the flat crystal grains having an average thickness in a direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio, which is the ratio of the grain width in a direction parallel to the surface to the average thickness, of 1.5 or more, and a crystal orientation that deviates from the Goss orientation by 10° or more, and in a cross section in the sheet thickness direction, a length of the grain boundary of the flat crystal grains accounts for 50% or more of the length of the interface between the silicon steel sheet and the oxide layer, and a plurality of linear thermal strains extending in a direction forming an angle of 80 to 100° with respect to the rolling direction are formed on the surface of the silicon steel sheet at intervals of 1.0 to 20.0 mm with respect to the rolling direction.
[2] In the grain-oriented electrical steel sheet according to [1], the average thickness of the flat crystal grains may be 0.5 to 2.0 μm.
[3] In the grain-oriented electrical steel sheet according to [1] or [2], the coverage of the oxide layer on the surfaces of the flat crystal grains constituting the interfaces may be 50% or more.
[4] A method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes a hot rolling step of heating and hot rolling a slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot rolling step, a pickling step of pickling the hot-rolled sheet after the hot-rolled sheet annealing step, a cold rolling step of cold-rolling the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet, a grinding step of grinding a surface of the cold-rolled sheet after the cold rolling step, a contacting step of contacting the cold-rolled sheet after the grinding step with an aqueous liquid having a pH of 4.0 to 10.0, a decarburization annealing step of decarburization annealing the cold-rolled sheet after the contacting step, and a finish annealing step of applying an annealing separator to the cold-rolled sheet after the decarburization annealing step, and then performing finish annealing to form an oxide layer consisting of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet to become a base steel sheet. the finish annealing step of forming an insulating coating layer on the surface of the oxide layer after the finish annealing step to obtain a grain-oriented electrical steel sheet comprising the silicon steel sheet, the oxide layer, and the insulating coating layer; and a magnetic domain control step of irradiating the surface of the grain-oriented electrical steel sheet after the insulating coating forming step with a laser, an electron beam, or plasma to form a plurality of linear thermal distortions extending in a direction forming an angle of 80 to 100° with respect to the rolling direction on the surface of the silicon steel sheet, the linear thermal distortions each having an interval of 1.0 to 20.0 mm in the rolling direction. In the grinding step, grinding is performed using abrasive grains having a Knoop hardness of 1000 or more or abrasive paper, roll, or brush to which the abrasive grains are fixed, so that the grinding amount of the cold-rolled sheet is 0.10 to 3.00 g/m ² on at least one surface.

The above aspect of the present invention makes it possible to provide a grain-oriented electrical steel sheet with excellent magnetic properties and a method for manufacturing the same.

1 is a schematic diagram of a cross section of a grain-oriented electrical steel sheet according to an embodiment of the present invention. FIG. FIG. 2 is a diagram for explaining a method for measuring the average thickness and aspect ratio of crystal grains. FIG. 2 is a diagram illustrating a method for measuring the coverage of an oxide layer on a flat crystal grain.

The following describes a grain-oriented electromagnetic steel sheet according to one embodiment of the present invention (grain-oriented electromagnetic steel sheet according to this embodiment) and its manufacturing method.

<Grain-oriented electrical steel sheet>
As shown in FIG. 1 , the grain-oriented electrical steel sheet 1 according to this embodiment has a silicon steel sheet 11 (hereinafter sometimes referred to as a base steel sheet, or simply as a steel sheet), an oxide layer 21 made of one or more oxides of Mg, Al, and Si formed on the surface of the silicon steel sheet 11, and an insulating coating layer 31 formed on the surface of the oxide layer 21.
The oxide layer 21 and the insulating coating layer 31 may be formed on only one side of the steel sheet, but it is preferable to form them on both sides from the viewpoint of insulation properties, etc. Each will be described below.

[Silicon steel sheet]
(One or more oxides of Mg, Al, and Si with a circle equivalent diameter of 0.1 to 3.0 μm are present at a density of 0.010 to 0.200 pieces/ ^μm2 within a range of 5 μm in the plate thickness direction from the interface between the silicon steel plate and the oxide layer)
In grain-oriented electrical steel sheets, by suppressing the decomposition and oxidation of inhibitors (precipitates present at grain boundaries, such as AlN) during final annealing and allowing them to exist up to high temperatures, it becomes possible to accumulate a high degree of Goss orientation during secondary recrystallization, in other words, to accumulate crystals closer to the ideal Goss orientation, thereby improving magnetic flux density and reducing iron loss.
The size of the precipitates that become inhibitors is very small, ranging from several tens of nm to about 100 nm in circle equivalent diameter. There is also a size distribution. When there is a size distribution, the decomposition and oxidation of small-sized inhibitors is completed at low temperatures, and the inhibitor effect is lost. In that case, secondary recrystallization in the Goss orientation closer to the ideal Goss orientation becomes difficult, and it is difficult to improve the magnetic flux density. On the other hand, the above problem can be solved by controlling the size distribution of the inhibitors to a constant value (so that the size difference is small), but this is extremely difficult industrially.
On the other hand, if the inhibitor can be made to exist up to high temperatures by suppressing decomposition and oxidation by some method even in a state where the size distribution of the inhibitor occurs, secondary recrystallization of crystal grains closer to the ideal Goss orientation can be caused. In addition, a method of using a highly heat-resistant inhibitor can be used to suppress the decomposition and oxidation of the inhibitor. On the other hand, as a method of achieving this without changing the components of the inhibitor, it is known that oxide particles of Si (hereinafter referred to as Si-based pre-oxides) formed in the steel sheet surface or near the surface in the decarburization annealing process contribute. Although the mechanism is speculation, it is thought that the oxidation of the inhibitor occurs when a small amount of oxygen contained in the finish annealing atmosphere oxidizes AlN and the like on the steel sheet surface, and that the above-mentioned Si-based pre-oxides prevent and reduce the oxidation.
However, the Si-based pre-oxides tend to be formed unevenly at various locations on the surface of the silicon steel sheet, and if the formation is uneven, the inhibitor's effect of suppressing decomposition and oxidation varies depending on the location on the steel sheet surface, making it difficult to obtain the intended effect.

The present inventors have investigated the cause of the non-uniform formation of the oxide layer at each site on the surface after finish annealing, and have found that Fe-based oxides and reaction products between the surface metal of the steel sheet and oiliness agents or extreme pressure additives contained in the rolling oil used during cold rolling are present non-uniformly on the surface of the steel sheet before decarburization annealing, and that these Fe-based oxides and reaction products prevent the Si-based pre-oxides on the steel sheet surface from being densely, thickly and uniformly formed in a certain thickness region from the surface during decarburization annealing.
Since it is difficult to make the Fe-based oxide film and reaction products uniform during cold rolling, the present inventors have conducted research into neutralizing the factors inhibiting the formation of the Si-based pre-oxides. As a result, as described below, the inventors have found that by constantly grinding the surface (at least one side) of the cold-rolled sheet before the decarburization annealing process using abrasive grains or abrasive paper, roll or brush with abrasive grains fixed thereon to expose a clean metal surface, and then immediately contacting the surface with an aqueous liquid, the Fe-based oxides and reaction products that are factors inhibiting the formation of the Si-based pre-oxides can be removed from the surface of the steel sheet, and the Si-based pre-oxides can be uniformly formed in a region of a certain thickness from the surface of the steel sheet after the decarburization annealing process. Therefore, based on these findings, optimization was performed in a magnetic domain control material irradiated with a laser, an electron beam, plasma, etc.
Based on the above findings, in the grain-oriented electrical steel sheet 1 according to this embodiment, one or more oxides of Mg, Al and Si (oxide particles 101) having a circle equivalent diameter of 0.1 to 3.0 μm are present at a density of 0.010 to 0.200 particles/μm ² within a range of 5 μm in the sheet thickness direction from the interface between the silicon steel sheet 11 and the oxide layer 21. The oxides may be one or more oxides of Mg, Al and Si (including composite oxides), but when the manufacturing conditions described below are assumed, they are often oxides containing Mg, Al and Si, such as spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ) and mullite ( _2SiO 2.3Al ₂ O ₃ ).
If the oxide number density is too low, the adhesion of the oxide layer to the steel sheet will be poor, and the formation of flat crystal grains described below will be non-uniform. On the other hand, if the oxide number density is too high, the area occupied by the metal part of the steel sheet will be small, and the magnetic flux density will decrease. In addition, the proportion of flat crystal grains will be relatively small, making it difficult to achieve the effect of reducing iron loss.
By uniformly forming the Si-based pre-oxides in a predetermined region after decarburization annealing, the variation in the inhibitor decomposition/oxidation suppression effect in the steel sheet during final annealing is reduced, and the magnetic flux density is improved in the grain-oriented electrical steel sheet. In addition, by appropriately forming flat crystal grains, the 180° magnetic domain width is reduced, which corresponds to a high magnetic flux density even when used as a magnetic domain control material, and a greater iron loss reduction effect is obtained.

(On the surface side of the silicon steel sheet, there are flat crystal grains with an average thickness in the direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio, which is the ratio of the grain width in the direction parallel to the surface to the average thickness, of 1.5 or more, and a deviation of the crystal orientation from the Goss orientation of 10° or more.)
(In the cross section in the plate thickness direction, the length of the grain boundary of the flat crystal grains accounts for 50% or more of the length of the interface between the silicon steel plate and the oxide layer)
As described above, in the grain-oriented electrical steel sheet according to this embodiment, the decarburization annealing process is mainly used to uniformly form Si-based pre-oxides in the surface layer (within 5 μm from the surface) of the silicon steel sheet (base steel sheet), thereby suppressing the decomposition and oxidation of the inhibitor during the final annealing and allowing it to exist up to high temperatures. In this case, it is possible to highly accumulate the Goss orientation, that is, to accumulate crystals closer to the ideal Goss orientation, resulting in improved magnetic flux density. In other words, it is possible to reduce iron loss.
On the other hand, inducing secondary recrystallization at a higher temperature means that only crystal grains closer to the ideal Goss orientation undergo secondary recrystallization. In this case, the number of Goss-oriented grains undergoing secondary recrystallization decreases, and the number of Goss-oriented grains per unit area of the steel sheet decreases. In other words, the crystal grain size per Goss-oriented grain becomes larger.
The iron loss required for grain-oriented electrical steel sheets is divided into hysteresis loss and eddy current loss. Hysteresis loss is reduced by improving the magnetic flux density. On the other hand, there are two types of eddy current loss: classical eddy current loss, which is reduced by reducing the sheet thickness and increasing the resistivity of the steel sheet, and abnormal eddy current loss, which is reduced by reducing the magnetic domain width formed in the Goss-oriented grains. Since the reduction in sheet thickness and the increase in the resistivity of the steel sheet in reducing classical eddy current loss often affect productivity, it is important to reduce abnormal eddy current loss, i.e., to reduce the magnetic domain width. The magnetic domain width is generally correlated with the crystal grain size of the Goss orientation. In general, the magnetic domain width of the so-called 180° magnetic domain generated in the grain-oriented electrical steel sheet is also reduced by reducing the grain size.
In other words, although the magnetic flux density can be improved by controlling the above oxides, the coarsening of the crystal grain size increases abnormal eddy current loss, and there is a concern that the iron loss reduction effect commensurate with the improvement in magnetic flux density cannot be obtained. The same is true for magnetic domain control materials created by irradiation with laser, electron beam, plasma, etc.

Therefore, the inventors have studied a method of reducing iron loss commensurate with the improvement of magnetic flux density, that is, a method of reducing abnormal eddy current loss in the case where the frequency of ideal Goss orientation crystal grains is increased and magnetic domain control is assumed, that is, a method of reducing magnetic domain width. As a result, it was found that, as described above, secondary recrystallization is caused at a higher temperature, and only crystal grains closer to the ideal Goss orientation undergo secondary recrystallization, and even when the crystal grain size is large, by making crystal grains (flat crystal grains) that are flat and have a deviation angle of 10° or more from the Goss orientation present on the surface of the steel sheet, it is possible to energetically control the 180° magnetic domain width to a small state, and to suppress an increase in eddy current loss. This effect is also exhibited in magnetic domain control materials created by irradiation with laser, electron beam, plasma, etc. Specifically, as shown in FIG. 1, when flat crystal grains 102 are present on the surface side of the base steel sheet (silicon steel sheet) 11, the average thickness in the direction perpendicular to the surface is 0.5 to 5.0 μm, the aspect ratio, which is the ratio of the grain width in the direction parallel to the surface to the average thickness, is 1.5 or more, and the crystal orientation is deviated from the Goss orientation by 10° or more (the flat crystal grains are present as grains that constitute the outermost layer of the silicon steel sheet), eddy current loss is reduced even before magnetic domain control, and it has been found that the effect can be enjoyed even after magnetic domain control.
Crystal grains with an average thickness of less than 0.5 μm, an aspect ratio of less than 1.5, or a deviation from the Goss orientation of less than 10° cannot sufficiently reduce the magnetic domain width, and therefore cannot sufficiently reduce core loss.
On the other hand, since the crystal grains are deviated from the Goss orientation, if the average thickness of the crystal grains exceeds 5.0 μm, the overall magnetic properties deteriorate, that is, the magnetic flux density decreases and the core loss increases.
The average thickness of the flat crystal grains is preferably 0.5 to 2.0 μm in order to fully obtain the effect of reducing the magnetic domain width when thermal distortion is applied by laser, electron beam, plasma irradiation, etc., which can reduce the magnetic domain width in the first place.

In order to fully obtain the above-mentioned magnetic domain refinement effect, the length of the grain boundary of the flat crystal grains accounts for 50% or more of the length of the interface between the base steel sheet and the oxide layer in the cross section in the sheet thickness direction.
If the ratio of flat crystal grains constituting the interface is small, the effect of reducing the magnetic domain width is insufficient, and therefore a sufficient effect of reducing iron loss cannot be obtained.

In the manufacturing method of grain-oriented electrical steel sheet, during finish annealing, tiny Goss orientation grains inside the thickness direction of the steel sheet grow while encroaching on surrounding crystal grains having orientations other than the Goss orientation, and as a result, the proportion of Goss orientation grains (crystal grains in which the longitudinal direction of the silicon steel sheet is the <100> direction and the surface direction is the <110> direction) increases from the inside of the thickness direction to the surface of the sheet, and further in the rolling direction and width direction.
In the grain-oriented electrical steel sheet according to this embodiment, oxides are present discretely in the surface layer of the steel sheet. As a result, during grain growth of the Goss-oriented grains present inside the sheet thickness, tiny flat crystal grains remain in the surface layer of the steel sheet without being encroached upon by the Goss-oriented grains, resulting in the formation of "flat crystal grains" which are confirmed as flat grains.

The average thickness, aspect ratio, and deviation of crystal orientation of crystal grains present in the surface layer portion (within 5 μm from the interface) of the silicon steel sheet can be measured by the following method.
A sample of, for example, about 20 mm square is cut out from the steel sheet so that a surface parallel to the rolling direction (RD direction) is obtained as a cross section, and the cross section is polished to a mirror surface. In addition, since it is difficult to measure the crystal orientation when the steel sheet is distorted by polishing, a polishing material such as colloidal silica is used in the final polishing process to prepare a polished sample so that no distortion is introduced. The polished sample is used to observe the cross-sectional shape with an FE-SEM, and then the crystal orientation is measured by EBSD measurement. For the FE-SEM, "SU-70" (manufactured by Hitachi High-Tech Corporation) is used as an example, and for the EBSD measurement, "Digiview" (manufactured by TSL Solutions) is used as an example. Specific examples of the method include the following. The cross section is observed with an FE-SEM at a magnification of 500 times, and an electron microscope image is obtained. The interface between the insulating coating layer and the oxide layer and the interface between the oxide layer and the steel sheet, which will be described later, are identified from the difference in electron density in the electron microscope image. When identifying the above-mentioned interface, if the FE-SEM is equipped with an elemental analyzer (EDS), it is possible to identify the interface more precisely from differences in elemental species, such as P, B, O, and Fe, contained in the insulating coating layer, the oxide layer, and the silicon steel plate.
Next, the crystal orientation of the steel sheet is measured by EBSD on the cross section of the same field of view. Specifically, in a 500x field of view where it is assumed that 100 or more flat grains are included, the crystal orientation is measured at measurement point pitches of 0.25 μm in an area with a cross-sectional length of 200 μm in the rolling direction and 70 μm in the sheet thickness direction. The boundary where the crystal orientation difference is 15° or more is defined as a grain boundary, and the area surrounded by this grain boundary is defined as a grain. If the number of grains in the field of view is less than 100, measurements are made on additional fields of view.
Regarding these crystal grains, the average thickness of the crystal grains is determined by the methods shown in a) to d) in FIG.
a) Draw an imaginary line (1) in the thickness direction (normal direction) of the steel plate to determine both ends of the grain.
b) Draw imaginary lines (2) in the thickness direction at 2.5% of the distance L between both ends of the crystal grain (the lines between them indicate 95% of the grain width).
c) Between the imaginary lines drawn in b) above (95% width portion of the crystal grain), draw an average line (3) on the envelope of the interface between the crystal grain and the oxide layer and on the lower side of the crystal grain (the grain boundary on the opposite side to the oxide layer).
d) The distance between the two average lines drawn in c) above is calculated as the thickness t(4) (average of both ends, the center, and the midpoint between both ends and the center, a total of five points).
The range between both ends of the grain drawn in the above step a) is regarded as the width of this crystal grain, and the aspect ratio is calculated.

The crystal orientation of the Fe ferrite phase is measured for all of the above crystal grains with an average thickness of 0.5 to 5.0 μm and an aspect ratio of 1.5 or more. The measured crystal orientation is then used to obtain a crystal orientation map called an IPF map, which shows the crystal orientation relative to the rolling direction (RD direction) and the normal direction of the steel sheet surface (ND direction). The average orientation difference between each crystal grain and the Goss orientation is calculated, and this is taken as the deviation from the Goss orientation. If the deviation from the Goss orientation is 10° or more, the grain is considered to be a flattened grain.

The average (simple average) of the average thickness of the flat crystal grains is obtained by dividing the sum of the average thicknesses of each flat crystal grain obtained above by the number of flat crystal grains.

Since the flat crystal grains are flat in the rolling direction (longitudinal direction) and width direction, any method may be used to observe the cross section in the plate thickness direction, but the method of obtaining a cross section of the above-mentioned steel plate parallel to the rolling direction (RD direction) and obtaining a crystal orientation map by EBSD to confirm the presence of "flat crystal grains" is preferable because it has high accuracy. Another method of simply confirming the presence of "flat crystal grains" is to polish a surface parallel to the rolling direction (RD direction) to obtain a smooth cross section, and then to confirm by a method such as the so-called nital method (nitric acid ethanol method, described in JIS-G-0553 (2019)), which reveals the crystal grain boundaries. However, this method does not identify the crystal orientation, and it is necessary to measure the crystal orientation separately using EBSD, etc., so in this embodiment, the method of using the above-mentioned FE-SEM and EBSD in combination is the most suitable.

The ratio of the length of the grain boundary of the flat crystal grains to the length of the interface between the base steel sheet and the oxide layer can be determined by the following method.
For example, in a field of view observed at a magnification of 500 times, SEM observation and EBSD measurement are performed on an area of 200 μm in cross-sectional length in the rolling direction at the interface between the silicon steel sheet and the oxide layer. This is performed at five locations, that is, over an interface length of 1000 μm. The proportion (percentage) of the grain boundaries of flat crystal grains with an average thickness of 0.5 to 5.0 μm, an aspect ratio of 1.5 or more, and an orientation difference from the Goss orientation of 10° or more is measured within the length (1000 μm) of the interface between the silicon steel sheet and the oxide layer.
Identification of the insulating coating layer, oxide layer, interface of the silicon steel plate, flat crystal grains, etc. can be performed in the same manner as above.
In the measurement, the length of the measurement range B where an oxide layer is formed on the surface of the silicon steel sheet is defined as B' (if the oxide layer is formed over the entire measurement range, B = B'), and the lengths of the parts where flat crystal grains are formed on the outermost layer of the silicon steel sheet and the interface between the silicon steel sheet and the oxide layer are the grain boundaries of the flat crystal grains are defined as b1, b2...bi (i = 3 in the figure), and the total length of b1 to bi (Σbi) is divided by the length where an oxide layer is formed on the surface of the silicon steel sheet by B'(Σbi/B') to measure the ratio of the length of the grain boundaries of the flat crystal grains to the length of the interface between the base steel sheet and the oxide layer.

(Thermal distortion)
By periodically forming linear thermal strain extending in a direction intersecting the rolling direction in the rolling direction, magnetic domain control can be performed. In the grain-oriented electrical steel sheet according to this embodiment, thermal strain is imparted to the steel sheet after the insulating coating formation step in the production of the grain-oriented electrical steel sheet.
Specifically, a coating liquid containing an insulating coating component having a tension-imparting function is applied to the steel sheet after the finish annealing, and then annealing is performed to bake the coating and flatten the steel sheet. After the baking and flattening annealing, a thermal strain is imparted to the steel sheet.
The thermal strain is a linear thermal strain extending in a direction of 80 to 100° with respect to the rolling direction of the grain-oriented electrical steel sheet. A plurality of such thermal strains are periodically present in the rolling direction, and the distance between adjacent thermally strained regions in the rolling direction is 1.0 to 20.0 mm. It is preferable that the thermal strains are approximately parallel to each other and are equally spaced in the rolling direction. The distance between thermally strained regions is the distance from the center of a thermally strained region to the center of an adjacent thermally strained region. Thermal strain can be imparted by laser, electron beam, or plasma irradiation, as described below.

The effect of reducing abnormal eddy current loss due to the flat crystal grains described above is also exhibited when magnetic domain control is performed by forming a thermally strained region. Furthermore, it has a secondary effect in the magnetic domain control of the thermally strained region as follows.
That is, forming oxides in a constant thickness region from the surface and as uniformly as possible over the entire surface of the steel sheet results in a more uniform color distribution on the surface. Also, forming oxides in a constant thickness direction on the surface layer and surface of the steel sheet results in increasing and uniforming the emissivity over the entire surface of the steel sheet. That is, when thermal distortion is applied, the energy is uniformly and easily absorbed by irradiation with a laser or electron beam, which makes it possible to reduce iron loss and variation, and as a result, the magnetic domain width can be reduced at each location on the steel sheet, which makes it possible to reduce iron loss.
In other words, in a steel sheet in which flat crystal grains are controlled, by controlling magnetic domains through the application of thermal strain, a synergistic effect is achieved that is greater than when each of these is performed alone.

The chemical composition of the base steel sheet (silicon steel sheet) is not limited, and may be the same as that of the base steel sheet of a known grain-oriented electrical steel sheet. For example, the composition may be within the range described below.
The chemical composition of the base steel sheet is, in mass%, 2.00 to 6.00% Si, with the remainder being Fe and impurities. This chemical composition is for controlling the crystal orientation to a Goss texture that is concentrated in the {110}<001> orientation, and ensuring good magnetic properties. There are no particular limitations on the other elements, and known elements may be included within known ranges in place of Fe. The typical content ranges (mass%) of typical elements other than Si are as follows:
C: 0 to 0.0050%
Mn: 0 to 1.0%
S: 0 to 0.0150%
Se: 0 to 0.0150%
Al: 0 to 0.0650%
N: 0 to 0.0050%
Cu: 0 to 0.40%
Bi: 0 to 0.010%
B: 0 to 0.080%
P: 0 to 0.50%
Ti: 0 to 0.0150%
Sn: 0 to 0.10%
Sb: 0 to 0.10%
Cr: 0 to 0.30%
Ni: 0 to 1.0%
Nb: 0 to 0.030%
V: 0 to 0.030%
Mo: 0 to 0.030%
Ta: 0 to 0.030%
W: 0 to 0.030%
These selective elements may be contained according to the purpose, so there is no need to limit the lower limit, and they may not be contained substantially. Furthermore, even if these selective elements are contained as impurities, the effects of the present invention are not impaired. Impurities refer to elements that are unintentionally contained, and refer to elements that are mixed in from raw materials such as ores and scraps, or the manufacturing environment, when industrially manufacturing the base steel sheet.

The chemical composition of silicon steel sheet is determined by dissolving the base steel sheet with hydrochloric acid or the like to obtain a solution. Then, a calibration curve is obtained by ICP (inductively coupled plasma) analysis of each element solution whose concentration is already known, and the obtained solution is then analyzed to quantitatively determine the contained elements.
In the case of grain-oriented electrical steel sheets in which an oxide layer and an insulating coating layer are formed on the surface of the base steel sheet (silicon steel sheet), the oxide layer and the insulating coating layer are removed before measurement.
The insulating coating layer can be removed by immersing the grain-oriented electrical steel sheet in an aqueous sodium hydroxide solution containing 30 to 50 mass % NaOH and 50 to 70 mass % H ₂ O at 80 to 90° C. for 7 to 10 minutes.
The grain-oriented electrical steel sheet from which the insulating coating layer has been removed is washed with water, and then dried for slightly less than one minute with a hot air blower. The grain-oriented electrical steel sheet after drying (grain-oriented electrical steel sheet without the insulating coating layer) can be immersed in an aqueous hydrochloric acid solution containing 10 mass % HCl at 80 to 90°C for 1 to 10 minutes to remove the oxide layer.
After immersion, the base steel sheet is rinsed with water and then dried for just under one minute with a hot air blower, whereby the base steel sheet (silicon steel sheet) can be removed from the grain-oriented electrical steel sheet having the oxide layer and the insulating coating layer.

(Thickness)
The thickness of the silicon steel sheet (base steel sheet) of the grain-oriented electrical steel sheet according to this embodiment is not limited, but considering the iron loss value, it is preferably 0.15 to 0.35 mm. If the sheet thickness exceeds 0.35 mm, the sheet thickness is large, so the classical eddy current loss described above increases, and the iron loss increases. On the other hand, if the sheet thickness is smaller than 0.15 mm, the rolling efficiency decreases, which is disadvantageous in terms of productivity and cost.

[Oxide layer]
In the grain-oriented electrical steel sheet according to this embodiment, an oxide layer made of one or more oxides of Mg, Al, and Si is formed on the surface of a silicon steel sheet (base steel sheet).
This oxide layer is formed during the final annealing by a solid-phase reaction between Mg and/or Al contained in the annealing separator and the Si-based pre-oxide formed on the steel sheet surface. For example, when an annealing separator containing MgO is used, a forsterite (Mg ₂ SiO ₄ ) coating layer is mainly formed as the oxide layer. Furthermore, AlN contained in the steel as an inhibitor is oxidized by oxygen in the annealing atmosphere on the surface of the silicon steel sheet in the latter half of the final annealing. As a result, spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), or mullite (2SiO ₂ .3Al ₂ O ₃ ) is generated. When an annealing separator consisting essentially of MgO is used, it is mostly generated as spinel (MgAl ₂ O ₄ ).
The oxide layer covers the surfaces of the flat crystal grains, thereby improving the adhesion to the insulating coating layer applied thereon. To obtain a sufficient effect, it is preferable that the coverage of the oxide layer on the flat crystal grains is 50% or more.

The coverage rate can be determined by the following method. That is, the presence of flat crystal grains is identified by EBSD as described above. Then, attention is paid to FE-SEM images of each flat crystal grain or elemental analysis images obtained by performing elemental analysis using EDS or the like based on the FE-SEM images. The length of the flat crystal grain where one or more oxide layers of Mg, Al, and Si exist between the insulating coating layer and the flat crystal grain or in the projected portion from the surface side of the flat crystal grain toward the inside of the steel sheet is measured. The length ratio where the oxide layer exists per 1000 μm of the interface length between the oxide layer or insulating coating layer and the flat crystal grain is determined as a percentage.
For example, in the state shown in FIG. 3, the coverage rate (%) can be calculated by (A1+A2+A3)/(a1+a2+a3)×100.

[Insulating coating layer]
In the grain-oriented electrical steel sheet according to the present embodiment, an insulating coating layer is formed on the surface of the oxide layer (as an upper layer). This insulating coating layer is essential when the grain-oriented electrical steel sheet is used as a transformer. When the grain-oriented electrical steel sheet is used as a transformer, it is laminated, and if a short circuit occurs between the laminated steel sheets (silicon steel sheets), an eddy current occurs in the transformer iron core, which causes an increase in iron core loss. Therefore, an insulating coating layer is formed on the surface of the steel sheet to impart electrical insulation, thereby reducing the iron core loss of the transformer. In addition, by applying tension to the steel sheet in the insulating coating of the grain-oriented electrical steel sheet, the magnetic domain width can be reduced, and abnormal eddy current loss and therefore iron loss can be reduced.
In addition to the electrical insulation properties described above, the insulating coating of grain-oriented electrical steel sheets is also required to have various properties necessary for producing iron cores, such as corrosion resistance, heat resistance, and slipperiness. In consideration of these needs, for example, a coating type whose main components are phosphate and colloidal silica is used for the insulating coating. In addition, for the purpose of imparting greater tension to the steel sheet, a coating whose main component is aluminum borate or a coating made of aluminum borate and silica may be used. Either coating may be a known coating formed by applying a coating liquid in which the components contained therein are dissolved or dispersed to the surface of an oxide layer and baking it.

<Production Method>
The grain-oriented electrical steel sheet according to this embodiment can obtain its effects as long as it has the above-mentioned characteristics regardless of the manufacturing method, but a manufacturing method including the following steps is preferable because it can be stably manufactured.
(I) a hot rolling step of heating and hot rolling the slab into a hot-rolled sheet;
(II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot rolling step;
(III) a pickling process for pickling the hot-rolled sheet after the hot-rolled sheet annealing process;
(IV) a cold rolling step of cold rolling the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet;
(V) a grinding step of grinding the surface of the cold-rolled sheet after the cold rolling step;
(VI) a contacting step of contacting the cold-rolled sheet after the grinding step with an aqueous liquid having a pH of 4.0 to 10.0;
(VII) a decarburization annealing step of performing decarburization annealing on the cold-rolled sheet after the contact step;
(VIII) A finish annealing step in which an annealing separator is applied to the cold-rolled sheet after the decarburization annealing step, and then the cold-rolled sheet is finish annealed to become a silicon steel sheet (base steel sheet), and an oxide layer made of one or more oxides of Mg, Al, and Si is formed on the surface of the cold-rolled sheet;
(IX) an insulating coating forming step of forming an insulating coating layer on the surface of the oxide layer after the finish annealing step to obtain a grain-oriented electrical steel sheet including the silicon steel sheet, the oxide layer, and the insulating coating layer;
(X) A magnetic domain control process in which a surface of the grain-oriented electrical steel sheet after the insulating coating formation process is irradiated with a laser, an electron beam, or plasma to form a plurality of linear thermal distortions on the surface of the silicon steel sheet, the linear thermal distortions extending in a direction forming an angle of 80 to 100° with respect to the rolling direction, the linear thermal distortions being spaced apart from one another in the rolling direction by 1.0 to 20.0 mm.
The method for producing a grain-oriented electrical steel sheet according to this embodiment may further include any one or more of the following steps.
(XI) A nitriding process for increasing the nitrogen content of the cold-rolled sheet.

Among these, the manufacturing method of the grain-oriented electrical steel sheet according to the present embodiment is characterized by the grinding step, the contact step, and the magnetic domain control step, while the hot rolling step, the hot-rolled sheet annealing step, the cold rolling step, the decarburization annealing step, the nitriding treatment step, the finish annealing step, and the insulating coating formation step can be performed under known conditions.
Preferred conditions are described below. Even if conditions are not described, the reaction can be carried out under known conditions.

[Hot rolling process]
In the hot rolling process, a slab having a predetermined chemical composition (a chemical composition corresponding to the chemical composition of the silicon steel sheet of the grain-oriented electrical steel sheet according to this embodiment) is heated and hot rolled to form a hot-rolled sheet.
The conditions are not limited, but the slab heating temperature is, for example, 1000 to 1400°C.

The chemical composition of the slab to be subjected to hot rolling may be determined according to the desired chemical composition of the grain-oriented electrical steel sheet, taking into consideration changes in the chemical composition in each process.
When obtaining the chemical composition of the silicon steel sheet of the grain-oriented electrical steel sheet according to the above-mentioned preferred embodiment, it is preferable to use a slab having, for example, the following chemical composition.
An example of a chemical composition is one which contains, in mass %, C: 0.040 to 0.100%, Si: 2.00 to 4.00%, and further contains Al, Mn, Se, S, B, N, etc. in predetermined ranges as inhibitors to form AlN, MnS, MnSe, BN, and further contains elements such as Cu, Sn, Cr, Ni, Mo, Nb, Bi, Sb, etc. as necessary.

The method for obtaining the slab is not limited. For example, molten steel having a predetermined chemical composition may be melted and the molten steel may be used to produce the slab. The slab may be produced by a continuous casting method, or the molten steel may be used to produce an ingot and the ingot may be bloomed to produce the slab. Alternatively, the slab may be produced by other methods.
The thickness of the slab is not particularly limited, but is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. As the slab, a so-called thin slab having a thickness of 10 to 70 mm may be used.
A so-called hot-rolled sheet (hot-rolled steel sheet) is obtained by hot rolling. The thickness (finished thickness) of the hot-rolled sheet is not particularly limited. However, the hot-rolled sheet is annealed, pickled, and then cold-rolled. It is known that the so-called cold rolling reduction rate affects the magnetic properties of the grain-oriented electrical steel sheet, and the thickness of the hot-rolled sheet is selected taking into account the required cold rolling reduction rate for the final thickness. For example, when the final thickness is 0.20 to 0.30 mm, the finished thickness of the hot-rolled sheet is preferably in the range of 2.0 to 4.0 mm.

[Hot-rolled sheet annealing process]
In the hot-rolled sheet annealing process, the hot-rolled sheet after the hot rolling process is annealed. By carrying out such annealing treatment, recrystallization occurs in the steel sheet structure, making it possible to realize good magnetic properties.
In the hot-rolled sheet annealing process of this embodiment, the hot-rolled sheet manufactured through the hot rolling process may be annealed according to a known method. The means for heating the hot-rolled sheet during annealing is not particularly limited, and a known heating method can be adopted. For example, so-called continuous annealing may be used, or the hot-rolled sheet may be coiled and subjected to batch annealing. The annealing conditions are also not particularly limited, but for example, the hot-rolled sheet may be annealed for 10 seconds to 5 minutes in a temperature range of 900 to 1200 ° C. The atmosphere is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen.

[Pickling process]
In the pickling process, the hot-rolled sheet after the hot-rolled sheet annealing process is pickled to remove scale (oxides) formed on the surface during hot rolling and hot-rolled sheet annealing. In the pickling process of this embodiment, a known method is used. As the pickling solution, known acids such as hydrochloric acid, sulfuric acid, and nitric acid are used. In addition, known pickling inhibitors and pickling accelerators may be added to the pickling solution as necessary. Furthermore, before the steel sheet is brought into contact with the pickling solution, it is also possible to perform physical treatment such as shot blasting on the steel sheet before pickling in order to penetrate the pickling solution into the interface between the scale and the steel sheet and improve the pickling efficiency.

[Cold rolling process]
In the cold rolling process, the hot rolled sheet after the hot rolled sheet annealing process is pickled and cold rolled to obtain a cold rolled sheet. The cold rolling may be a single cold rolling (a series of cold rolling without intermediate annealing) or may be multiple cold rollings with intermediate annealing by interrupting the cold rolling and performing at least one or two or more intermediate annealings before the final pass of the cold rolling process.
The cold rolling conditions may be in accordance with known methods. The cold rolling reduction of grain-oriented electrical steel sheet has a large effect on its magnetic properties. In particular, the final rolling reduction has a large effect, and the final rolling reduction can be set within the range of 80 to 95%. The final rolling reduction is the cumulative rolling reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative rolling reduction of cold rolling after final intermediate annealing.
When intermediate annealing is performed, the steel sheet is held at a temperature of, for example, 800 to 1200°C for 5 to 180 seconds. The annealing atmosphere is not particularly limited, but it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen in order to prevent oxidation of the steel sheet. The annealing method may be either so-called continuous annealing or batch annealing in a coil shape, or other methods may be used. The number of times intermediate annealing is preferably three or less, taking into account the manufacturing cost.

[Grinding process]
In the grinding process, the surface of the cold-rolled sheet after the cold rolling process is ground. At that time, grinding is performed using abrasive grains having a Knoop hardness of 1000 or more, or abrasive paper, rolls, or brushes to which the abrasive grains are fixed. When grinding a coiled cold-rolled sheet, it is preferable in terms of productivity and quality to grind continuously using a sheet threading line. In that case, it is common to use a brush in which the abrasive grains are mainly fixed. Of course, it is also possible to use a plate-shaped cold-rolled sheet instead of a coil, in which case grinding can also be performed using abrasive paper or the like.
As described above, by allowing the inhibitors (precipitates present at grain boundaries, such as AlN) to exist at as high a temperature as possible during the final annealing, only crystal grains having a crystal orientation closer to the ideal Goss orientation are allowed to grow, thereby improving the magnetic flux density.
However, although the size of the inhibitor is very small, ranging from several tens to about 100 nm, there is a size distribution. When there is a size distribution, the small-sized inhibitor starts to decompose at low temperatures. In that case, secondary recrystallization of only crystal grains close to the Goss orientation (ideal Goss orientation) becomes difficult, making it difficult to improve the magnetic flux density. On the other hand, it is extremely difficult industrially to control the size of the inhibitor to a constant, preferred size (so that the size difference is small).
On the other hand, if the decomposition and oxidation of the inhibitor can be suppressed so that the inhibitor can exist up to high temperatures, secondary recrystallization can occur only in crystal grains closer to the Goss orientation. It is also known that the aforementioned Si-based pre-oxides formed in the base steel sheet (the cold-rolled sheet to be used) in the decarburization annealing process contribute to the suppression of the decomposition and oxidation of the inhibitor.
However, these Si-based pre-oxides are easily affected by the process before the decarburization annealing process, and the formation state of these pre-oxides tends to be non-uniform at each part of the steel sheet surface. If the formation state is non-uniform, the inhibitor's effect of suppressing decomposition and oxidation varies depending on the location on the steel sheet surface, and the intended effect cannot be obtained.

Therefore, in the manufacturing method of the grain-oriented electrical steel sheet according to this embodiment, in order to make the state of formation of the oxide layer after finish annealing as uniform as possible over a certain thickness region on the surface of the steel sheet, Fe-based oxides that have formed non-uniformly on the steel sheet surface due to cold rolling, etc., and products of reactions with the steel sheet surface, such as oiliness agents or extreme pressure additives, which prevent the uniform formation of such oxide layers, are removed from the steel sheet surface before decarburization annealing by grinding the steel sheet surface.
Specifically, at least one surface of the steel sheet is ground using abrasive grains having a Knoop hardness of 1000 or more, or abrasive paper, roll or brush to which the abrasive grains are fixed, and Fe-based oxide films and reactants are removed from the steel sheet surface. If the Knoop hardness is less than 1000, the hardness of the abrasive grains is insufficient for the steel sheet, making grinding difficult. Or the grinding efficiency is reduced. Also, if the maximum particle size of the abrasive grains is less than 30 μm, the particle size of the abrasive grains becomes relatively small compared to the roughness of the steel sheet surface, making grinding difficult or reducing the grinding efficiency, which is not preferable. On the other hand, if the maximum particle size exceeds 300 μm, the particle size of the abrasive grains becomes too large relative to the roughness of the steel sheet surface, making surface scratches more noticeable during grinding and reducing the quality of the product's appearance, which is not preferable. Although there is no upper limit for the Knoop hardness, hard abrasive grains tend to become brittle and are prone to problems such as poor grinding when continuously used with abrasive paper, rolls, brushes, etc. that contain the abrasive grains, so the hardness is preferably 8000 or less, and more preferably 5000 or less. Alumina (Knoop hardness: about 2000), silicon carbide (Knoop hardness: about 2500), boron nitride (Knoop hardness: about 5000), diamond (Knoop hardness: about 7000), etc. are mainly used as the abrasive grains.

Specifically, the process of grinding cold-rolled sheet will be explained using an example of a brush roll containing abrasive grains. A brush roll is a metal roll with a resin lining on the surface, and the abrasive grains are embedded in fibers made of acrylic resin or the like, which are then planted in the form of hairs on the resin layer on the roll surface. To explain using an example of application to a continuous sheet threading line, the steel sheet is ground using a brush roll at a sheet threading speed of approximately 20 to 200 mpm (meter per minute), and the steel sheet is ground by contacting the brush roll, which rotates in the opposite direction to the sheet threading direction, with the steel sheet while moving at the position where the steel sheet and the brush roll come into contact. When grinding a steel sheet with a brush roll, the steel sheet is sandwiched between the brush roll and an idle roll, and the brush roll is pressed down against the idle roll side with respect to the sheet threading line (pass line). The amount of pressing down at this time is considered to be approximately 1.0 to 5.0 mm. Brush rolls with a diameter of approximately 200 to 500 mm are usually used. If the brush is too small, the brush and abrasive grains will wear out quickly, and if it is too large, the metal roll will become too large and the equipment will become too large. The brush is rotated in the direction opposite to the passing direction of the steel plate to perform grinding, as described above. The passing speed of the steel plate is in the range of 20 to 200 mpm as described above, and in this case, a brush rotation speed of approximately 500 to 2000 mpm is considered to be suitable to keep the amount of grinding within the specified range. If the rotation speed is low, the amount of grinding will be small, and if the reduction amount is increased to increase the amount of grinding, the brush roll and the steel plate pass against each other, so the steel plate does not pass smoothly due to the friction between the steel plate and the brush roll, and small movements occur, which is called "chattering". "Chattering" is an extremely undesirable phenomenon that should be avoided because it causes uneven grinding of the steel plate surface. In addition, if the rotation speed of the brush roll is higher than 2000 mpm, the friction between the brush roll and the passing steel plate will become too large, and not only will the above-mentioned "chattering" occur, but the motor driving the brush roll will also be overloaded, which is undesirable.

In order to sufficiently remove the Fe-based oxide film and reaction products unevenly formed on the surface of the cold-rolled sheet, the grinding amount is set to 0.10 g/ ^m2 or more on at least one surface. While the Fe-based oxide film and reaction products are sufficiently removed from the steel sheet surface, the service life of the abrasive grains is shortened and sludge generation becomes significant due to grinding, and the processing of this sludge is time-consuming and causes defects on the steel sheet surface due to pressing, etc., so the grinding amount is set to 3.00 g/ ^m2 or less.
The amount of grinding can be confirmed from the difference in weight of the steel sheet before and after grinding. The amount of grinding is the amount of grinding per side, and when grinding is performed on both sides, the amount of grinding per both sides is calculated, and for convenience, this value is halved. From the viewpoint of removing the Fe-based oxide film and reaction products from the entire surface of the steel sheet, the preferable range of the amount of grinding is 0.30 g/ ^m2 or more and 3.00 g/ ^m2 or less.

[Contacting step]
In the contact step, after the grinding step and before the decarburization annealing step, the surface of the cold-rolled sheet is brought into contact with an aqueous liquid having a pH of 4.0 to 10.0. This removes the abrasive grains attached to the surface of the steel sheet and the steel sludge generated during grinding. The aqueous liquid may be ion-exchanged water. It may also contain minerals such as Ca and Mg, and may contain carbonic acid or silicic acid as a counter ion. It may also be a liquid in which about 0.01 wt % of an acid selected from sulfuric acid, nitric acid, phosphoric acid, carbonic acid, carboxylic acid, phosphonic acid, etc. is added and the pH is adjusted with an alkali metal or alkaline earth metal. In particular, carboxylic acid and phosphonic acid are highly effective in removing abrasive grains and sludge from the steel sheet. In the case of ion-exchanged water, from the viewpoint of preventing dissolution, its electrical conductivity is preferably 0.1 to 10 μS/cm.
If the pH is less than 4.0, the steel sheet surface is etched by the acidic aqueous solution, causing corrosion of the steel sheet. If the pH is more than 10.0, the alkaline aqueous solution acts to promote oxidation of the metal surface after grinding, reducing the effect of removing the Fe-based oxides that were unevenly formed on the steel sheet surface in the grinding process. In this case, the initial intended effect of uniformly forming an oxide layer and oxide particles after finish annealing cannot be sufficiently obtained.
For the above purpose, the contact time is preferably 0.1 to 60 seconds, more preferably 1 to 60 seconds, and even more preferably 5 to 60 seconds. The flow rate of the aqueous liquid is preferably 1 to 100 L/min.
Furthermore, by carrying out the contact step, abrasive grains and sludge can be removed from the surface of the steel sheet, and factors that inhibit the uniform formation of an oxide layer and oxide particles after the finish annealing can be avoided.
In the case where the contact step is carried out, it is carried out after the grinding step in consideration of the above-mentioned purpose.
The surface of the cold-rolled sheet may be brought into contact with the aqueous liquid during the grinding step, but the above-mentioned effect cannot be obtained unless the contacting step is carried out after the grinding step.

[Decarburization annealing process]
In the decarburization annealing process, the cold-rolled sheet after the grinding process is subjected to decarburization annealing, which removes (decarburizes) C, which adversely affects magnetic properties, from the steel sheet and causes primary recrystallization of the cold-rolled sheet.
The decarburization annealing conditions are not limited, but annealing is performed in a nitrogen-hydrogen mixed atmosphere for decarburization, with the oxygen potential increased by humidification. In addition, since it is necessary to form a primary recrystallized structure, the humidification temperature (dew point) is determined in terms of the annealing temperature required for recrystallization and the oxygen potential at which decarburization is possible at the annealing temperature. The annealing temperature is about 700 to 900°C, and since annealing is generally performed in a continuous annealing process, soaking is performed for about 60 seconds. As described above, since annealing is performed in a humidified atmosphere with a high oxygen potential for decarburization, it is known that Si contained in the steel forms a layered oxide on the steel sheet surface and oxide particles inside the steel sheet (hereinafter, as above, referred to as Si-based pre-oxide).

[Nitriding process]
In the nitriding process, the nitrogen content of the steel sheet is increased to increase the amount of nitrides, thereby promoting secondary recrystallization of crystal grains closer to the Goss orientation in the finish annealing process. In the nitriding process, the nitrogen content of the steel sheet after the nitriding process is preferably 0.015 to 0.050 mass%. The method of the nitriding process is not limited, and any known method may be used.
The nitriding step is not essential and may be omitted. If nitriding is performed, it is preferable to perform it between the decarburization annealing step and the finish annealing step.

[Finish annealing process]
In the final annealing process, an annealing separator is applied to the cold-rolled sheet after the decarburization annealing process (or after the nitriding process if nitriding has been performed), and the cold-rolled sheet is then finish-annealed to form an oxide layer made of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet which becomes the base steel sheet (silicon steel sheet).
Finish annealing is usually performed by winding the steel sheet into a coil and batch annealing because the annealing time is long. Since the steel sheet temperature rises to about 1200°C, an annealing separator is applied to prevent the coiled steel sheet from seizing. MgO is generally used as the annealing separator. By applying such an annealing separator and then performing finish annealing, Mg contained in the annealing separator and Si-based pre-oxide formed on the steel sheet surface in the decarburization annealing process undergo a solid-phase reaction, and an oxide layer consisting of one or more oxides of Mg and Si is formed on the surface of the cold-rolled sheet. For example, when an annealing separator containing MgO is used, a layer of forsterite (Mg ₂ SiO ₄ ) coating is mainly formed as the oxide layer. Furthermore, AlN contained in steel as an inhibitor is oxidized by oxygen in the annealing atmosphere on the surface of the steel sheet in the latter half of the finish annealing, and at that time, it forms spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), or mullite (2SiO ₂ .3Al ₂ O ₃ ).When an annealing separator consisting essentially of MgO is used, it forms almost entirely as spinel (MgAl ₂ O ₄ ).
In the final annealing process, the steel sheet is heated to cause secondary recrystallization of the primary recrystallized grains obtained in the decarburization annealing process, thereby obtaining crystal grains having the Goss orientation. By holding the steel sheet at an annealing temperature close to 1200°C for a predetermined time, precipitates in the steel, such as nitrides (e.g., AlN) and sulfides (e.g., MnS), which have completed their role as inhibitors, are removed (purified) so as not to adversely affect the magnetic properties.
In the method for producing a grain-oriented electrical steel sheet according to the present embodiment, the size of the inhibitor is controlled to be larger and more uniform than usual in the cold-rolled sheet to be subjected to finish annealing, so that secondary recrystallization occurs only in grains close to the Goss orientation (grains having an orientation close to the Goss orientation).
The conditions for the finish annealing are not limited, but for example, the temperature is raised from room temperature at a rate of 10 to 100°C/h, and in the temperature range of 900 to 1000°C, where secondary recrystallization in the Goss orientation generally occurs, the temperature is raised at a rate of 5 to 20°C/h to promote preferential growth (secondary recrystallization) in the Goss orientation, and then, as described above, the inhibitor that has completed its role is purified at around 1200°C (for example, 1150 to 1250°C).Then, the coil is slowly cooled in a non-oxidizing atmosphere such as hydrogen or nitrogen, and taken out of the furnace.

[Insulating film forming process]
After the final annealing process, an insulating coating layer is formed on the coil in the area that will become the edge of the steel sheet in the subsequent insulating coating formation process. Grain-oriented electrical steel sheets are laminated and used in transformer manufacturing, but if there is a short circuit between the laminations when the transformer is in operation, iron loss increases and the transformer may even burn out, so the insulating coating formation process is an important process. After the final annealing process, the annealing separator on the coil is removed by water washing or pickling, and an insulating coating layer is formed on the surface of the oxide layer that has formed on the steel sheet surface.
For example, the insulating coating layer can be formed by applying a coating solution containing phosphoric acid or a phosphate, colloidal silica, and chromic anhydride or a chromate to the cold-rolled sheet (base steel sheet + oxide layer) after the final annealing, and baking and drying at 300 to 950 ° C for 10 seconds or more. The atmosphere during baking is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the baking in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen. In addition, as the coating type, it is possible to use a coating solution mainly composed of boric acid and alumina sol instead of the above-mentioned phosphate, or a coating solution mainly composed of boric acid and aluminosilicate (kaolin mineral, etc.), etc., to form an insulating coating mainly composed of aluminum borate. The application of aluminum borate can impart a large tension to the steel sheet, thereby reducing iron loss. In addition, this process also plays a role in flattening the steel sheet that has been coiled by batch annealing in the above-mentioned final annealing by continuous annealing. That is, the insulating coating is applied and the coil-shaped steel sheet is continuously annealed at about 800°C while being applied with a certain tension to obtain a flat steel sheet. For this reason, it is sometimes called the flattening annealing process.
Through these steps, a grain-oriented electrical steel sheet including a base steel sheet (silicon steel sheet), an oxide layer, and an insulating coating layer can be obtained.

[Magnetic domain control process]
In the magnetic domain control process, the grain-oriented electrical steel sheet after the insulating coating formation process is irradiated with a laser, an electron beam or plasma to form a plurality of linear thermal distortions on the surface of the base steel sheet, extending in a direction forming an angle of 80 to 100° with respect to the rolling direction, with each linear thermal distortion spaced apart by 1.0 to 20.0 mm in the rolling direction.
By forming the above-mentioned thermally strained region on the surface of the grain-oriented electrical steel sheet, the magnetic domains are subdivided and iron loss is reduced. If the direction, interval, etc. of the thermal strain are outside the above ranges, sufficient effects cannot be obtained.
Thermal distortion can be imparted by irradiating a laser, an electron beam, plasma, or the like under conditions that do not melt the base steel sheet. The conditions are not limited, but for example, laser irradiation is performed using a continuous wave laser or a pulsed laser as the laser. For example, as described in Patent Document 1, it is preferable to control the average energy density to a range of 0.8 to 2.0 mJ/ ^mm2 .

Example 1
Molten steel containing 3.25 mass% Si, 0.13 mass% Mn, 0.006 mass% S, 0.050 mass% C, 0.025 mass% acid-soluble Al, and 0.007 mass% N was continuously cast to obtain a slab with a thickness of 300 mm.
The slab was heated at 1150° C. for 60 minutes in an electric furnace adjusted to a nitrogen atmosphere, and then roughly hot-rolled to obtain a steel plate having a thickness of 40 mm. The slab was then finish-rolled to obtain a hot-rolled plate having a thickness of 2.3 mm.
Thereafter, the hot-rolled sheet was annealed in a continuous annealing furnace adjusted to a nitrogen atmosphere by heating at 1100° C. for 60 seconds and then cooling.
The obtained steel sheet (hot-rolled sheet) was pickled with 10% hydrochloric acid to remove scale from the steel sheet.
Thereafter, cold rolling was carried out to obtain a cold-rolled sheet having a thickness of 0.22 mm.
The surface of the obtained cold-rolled sheet was ground using various brushes containing abrasive grains as shown in Table 1. After grinding, the surface was brought into contact with ion-exchanged water having a pH of 2.5 to 12.0. However, for comparison, some steel sheets were not ground and some steel sheets were not contacted with ion-exchanged water after grinding. The contact time was 5 seconds, and the flow rate of the aqueous liquid was 10 L/min.

A sample having a width of 1.0 m and a length of 1.0 m was taken from the steel sheet that had been ground and contacted with an aqueous liquid (the cold-rolled sheet after cold rolling if neither was performed, or the cold-rolled sheet after the grinding step if no contact with an aqueous liquid was performed), and the appearance of both sides of the sample was evaluated.
The criteria for judgment are as follows:
5: Very beautiful (no streaks in the sheet running direction)
4: Beautiful (several streaks in the strip running direction)
3: Some streaks present (~20 streaks in the strip running direction)
1: Streaks or unevenness on the entire surface. With the exception of a few cases, further evaluation was not carried out for cases with poor appearance (rating: 1).

In addition, the steel sheet that had been ground and contacted with an aqueous liquid (the cold-rolled sheet after cold rolling when neither was performed, or the cold-rolled sheet after the grinding step when no contact with an aqueous liquid was performed) was subjected to decarburization annealing under the following conditions. The annealing atmosphere was a 50% nitrogen + 50% hydrogen atmosphere with an oxygen potential of _PH2O / _PH2 = 0.30. The oxygen potential was adjusted by humidifying the atmosphere before introducing it into the furnace. In this atmosphere, decarburization annealing was performed by soaking at 850°C for 60 seconds.
Thereafter, the steel sheet was subjected to nitriding treatment by soaking in a nitrogen-hydrogen-ammonia atmosphere at 750° C. for 30 seconds. The ammonia concentration was adjusted so that the nitrogen content of the steel sheet after the nitriding treatment was N: 0.020 mass %.
Thereafter, an aqueous slurry of an annealing separator mainly composed of MgO was prepared, and the annealing separator was applied to both sides of the steel sheet so that the post-dry adhesion amount per side was 6 g/ ^m2 , and then dried. The composition of the annealing separator was 100 parts by mass of MgO, 5 parts by weight of _TiO2 , and 0.020% by mass of _FeCl2 was added as Cl.
Thereafter, for finish annealing, the samples were placed in a batch annealing furnace and heated at an average heating rate of 20°C/h in a 50% nitrogen + 50% hydrogen atmosphere. After heating up to 1200°C, the atmosphere was switched to 100% hydrogen and soaked for 20 hours, after which the temperature was lowered.
After the completion of the final annealing, the steel sheet was taken out of the furnace and the annealing separator was washed off with water. At this time, the surface of the steel sheet (silicon steel sheet) had a glass coating made of forsterite due to secondary recrystallization, and an oxide layer made of granular spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ) and/or mullite formed between the glass coating and the steel sheet.
This steel sheet (a steel sheet having a glass coating, which is an oxide layer, formed on the surface of a silicon steel sheet, which is a base steel sheet) was then coated with a chemical solution containing insulating coating components consisting of aluminum phosphate, colloidal silica, and chromic anhydride, and baked by heating to 800°C in a nitrogen atmosphere and holding for 30 seconds. At this time, the amount of the insulating coating layer attached was 4.8 g/ ^m2 per side. A grain-oriented electrical steel sheet was thus obtained.

The surface of the obtained grain-oriented electrical steel sheet (having a silicon steel sheet, a glass coating (oxide layer), and an insulating coating layer) was irradiated with a laser. A fiber laser with a laser output of 200 W was used, and the laser irradiation diameter was adjusted to φ0.2 mm and the irradiation energy density to 1.5 mJ/ ^mm2 . The scanning direction was set to 88° to the rolling direction of the steel sheet, and the irradiation pitch (the distance between thermal strains in the rolling direction) was set to 4.0 mm.

The number density of one or more oxides of Mg, Al and Si having a circle equivalent diameter of 0.1 to 3.0 μm within a range of 5 μm in the sheet thickness direction from the interface with the oxide layer of the silicon steel sheet of the obtained grain-oriented electrical steel sheet, the coverage of the oxide layer and the flat crystal grains were evaluated using the methods described above.
In this example, one or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm within a range of 5 μm from the interface with the oxide layer in the plate thickness direction were spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), and mullite ( _2SiO 2.3Al ₂ O ₃ ), that is, oxides containing Mg, Al, and Si.

<Magnetic property measurement>
In addition, 10 sheets measuring 500 mm in the sheet width direction × 500 mm in the rolling direction were taken from the obtained grain-oriented electrical steel sheet, and the magnetic flux density when excited with a magnetizing force of 800 A/m according to the single sheet magnetic property measurement method described in JIS-C-2556:2015 (hereinafter B8) and the iron loss when excited with an excitation frequency of 50 Hz and a magnetic flux density of 1.7 T (hereinafter W17/50) were measured.
If B8 was 1.90 T or more and W17/50 was 0.73 W/kg or less, it was determined that the material had excellent magnetic properties.

<Adhesion>
In addition, a sample of 300 mm in the rolling direction × 300 mm in the width direction was taken, and this sample was wound around a SUS304 round bar with a diameter of 20 mm (φ20 mm). After unwinding, the insulating coating in the recess on the inside where it was wound was observed to evaluate the adhesion of the insulating coating.
The criteria for judgment were as follows:
G (GOOD): No coating peeling P (POOR): Partial coating peeling B (BAD): Full coating peeling

As can be seen from Tables 1 to 3, in the examples where the surface of the steel sheet was ground and brought into contact with an aqueous liquid under the conditions of the present invention, one or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm were present at a density of 0.010 to 0.200 pieces/ ^μm2 within a range of 5 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer, and the length of the grain boundary of the flat crystal grains accounted for 50% or more of the length of the interface between the silicon steel sheet and the oxide layer in the cross section in the sheet thickness direction. Furthermore, as a result, these examples had excellent magnetic properties.
On the other hand, in the cases where the contact with the prescribed aqueous liquid was not performed or the grinding conditions were not favorable, the appearance required in general was not satisfied, or the oxide in the surface layer was not sufficiently formed or the flat crystal grains were not sufficiently formed, and as a result, the magnetic properties were inferior (some of the appearance defects were not evaluated).

Example 2
Using molten steel and slabs having the same components as those used in Example 1, hot rolling, hot-rolled sheet annealing, pickling, and cold rolling were carried out in the same manner as in Example 1 to produce cold-rolled sheets having a thickness of 0.22 mm.
The surface of the steel sheet was ground using various brushes containing abrasive grains as shown in Table 4, and then brought into contact with ion-exchanged water of pH 6.0. The contact time was 5 seconds, and the flow rate of the aqueous liquid was 10 L/min. Thereafter, decarburization annealing, nitriding treatment, application of an annealing separator, and finish annealing were performed in the same manner as in Example 1, and the annealing separator was removed by washing with water, and then an insulating coating layer was formed to obtain a grain-oriented electrical steel sheet.

The surface of the obtained grain-oriented electrical steel sheet (having a silicon steel sheet, a glass coating (oxide layer), and an insulating coating layer) was irradiated with a laser. A fiber laser with a laser output of 200 W was used, and the laser irradiation diameter φ was adjusted to 0.2 mm and the irradiation energy density was adjusted to 1.8 mJ/ ^mm2 . The scanning direction was set to 75 to 105° with respect to the rolling direction of the steel sheet, and the irradiation pitch (the distance between thermal strains in the rolling direction) was changed in the range of 0.5 to 25.0 mm.
The number density of one or more oxides of Mg, Al and Si having a circle equivalent diameter of 0.1 to 3.0 μm within a range of 5 μm in the sheet thickness direction from the interface with the oxide layer of the silicon steel sheet of the obtained grain-oriented electrical steel sheet, the coverage of the oxide layer and the flat crystal grains were evaluated in the same manner as in Example 1.
In this example, one or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm within a range of 5 μm from the interface with the oxide layer in the plate thickness direction were spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), and mullite ( _2SiO 2.3Al ₂ O ₃ ), that is, oxides containing Mg, Al, and Si.

<Magnetic property measurement>
The evaluation was carried out in the same manner as in Example 1.

<Adhesion>
The measurement was carried out in the same manner as in Example 1.

As can be seen from Tables 4 to 6, if the laser irradiation conditions are outside the range of the present invention, it is not possible to obtain sufficiently low iron loss.

The present invention provides a grain-oriented electrical steel sheet with excellent magnetic properties and a manufacturing method thereof. Therefore, it has high industrial applicability.

REFERENCE SIGNS LIST 1 Grain-oriented electrical steel sheet 11 Silicon steel sheet 21 Oxide layer 31 Insulation coating layer 101 Oxide particle 102 Flat crystal grain

Claims

A silicon steel sheet;
an oxide layer formed on a surface of the silicon steel sheet and made of one or more oxides of Mg, Al, and Si;
an insulating coating layer formed on a surface of the oxide layer;
having
One or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm are present at a density of 0.010 to 0.200 pieces/ μm2 within a range of 5 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer of the silicon steel sheet,
The silicon steel sheet has flat crystal grains on a surface side thereof, the flat crystal grains having an average thickness in a direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio, which is the ratio of the grain width in a direction parallel to the surface to the average thickness, of 1.5 or more, and a crystal orientation deviation from the Goss orientation of 10° or more;
In the cross section in the plate thickness direction, the length of the grain boundary of the flat crystal grains accounts for 50% or more of the length of the interface between the silicon steel plate and the oxide layer;
A plurality of linear thermal strains extending in a direction forming an angle of 80 to 100° with respect to the rolling direction are formed on the surface of the silicon steel plate at intervals of 1.0 to 20.0 mm with respect to the rolling direction.
The grain-oriented electrical steel sheet is characterized in that
The average thickness of the flat crystal grains is 0.5 to 2.0 μm.
The grain-oriented electrical steel sheet according to claim 1 .
The coverage of the oxide layer on the surface of the flat crystal grain constituting the interface is 50% or more.
The grain-oriented electrical steel sheet according to claim 1 or 2.
a hot rolling step of heating and hot rolling the slab into a hot rolled sheet;
A hot-rolled sheet annealing process for annealing the hot-rolled sheet after the hot rolling process;
A pickling process for pickling the hot-rolled sheet after the hot-rolled sheet annealing process;
A cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet;
A grinding step of grinding the surface of the cold-rolled sheet after the cold rolling step;
A contacting step of contacting the cold-rolled sheet after the grinding step with an aqueous liquid having a pH of 4.0 to 10.0;
a decarburization annealing step of performing decarburization annealing on the cold-rolled sheet after the contact step;
A finish annealing process in which an annealing separator is applied to the cold-rolled sheet after the decarburization annealing process, and then the cold-rolled sheet is finish annealed to form an oxide layer composed of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet which becomes a base steel sheet;
an insulating coating forming step of forming an insulating coating layer on a surface of the oxide layer after the final annealing step to obtain a grain-oriented electrical steel sheet including the silicon steel sheet, the oxide layer, and the insulating coating layer;
a magnetic domain control process in which a surface of the grain-oriented electrical steel sheet after the insulating coating formation process is irradiated with a laser, an electron beam, or plasma to form a plurality of linear thermal distortions on the surface of the silicon steel sheet, the linear thermal distortions extending in a direction forming an angle of 80 to 100° with respect to the rolling direction, the linear thermal distortions being spaced apart from each other by 1.0 to 20.0 mm in the rolling direction;
Equipped with
In the grinding step, grinding is performed using abrasive grains having a Knoop hardness of 1000 or more or abrasive paper, roll, or brush to which the abrasive grains are fixed, so that the grinding amount of the cold-rolled sheet is 0.10 to 3.00 g/m 2 on at least one surface.
A method for producing a grain-oriented electrical steel sheet.